1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a floating body germanium (Ge) phototransistor with a photo absorption threshold bias region, and an associated fabrication process.
2. Description of the Related Art
FIG. 1 is a Cross-Sectional Transmission Electron Microscopy (XTEM) image of an interface between Ge and Si3N4. The Ge film has been regrown by liquid phase epitaxy. In a co-pending application entitled METHOD OF EPITAXIAL GROWTH OF GERMANIUM PHOTODETECTOR FOR CMOS IMAGER, Lee et al., Ser. No. 11/069,424, filed Feb. 28, 2005, a method for growing single crystalline Ge film by liquid phase epitaxy is presented, along with a means of fabricating a PIN photo detector for infrared photon detection. However, it is noted that the interface between Ge single crystal and the bottom dielectric layer (Si3N4) is not perfect, as shown in the TEM image. This interface may potentially cause leakage current in the diode. This interface is likely the result of the Ge PIN diode using the entire Ge film, so that the bottom interface deteriorates the electrical performance.
It is advantageous that a thicker germanium film be used for the transistor fabrication, so that the drain depletion region does not contact the high defect region of the germanium that is adjacent to the insulator interface. To address the floating body issue, the source electrode can be extended through the full thickness of germanium thin film, extending to the silicon substrate. The source can also be extended through the high defect density germanium, to insulator interface. However, with this structure the source junction leakage current is relatively high. As a result, the floating substrate effect of the transistor is diminished.